The invention relates generally to the field of advanced static seals. More specifically, the invention relates to methods for depositing abradable material onto a seal backing material to form air seals that reduce clearances between rotating and stationary parts in turbines.
Improving sealing between rotating and stationary parts in turbines can significantly reduce parasitic leakages thereby improving performance. Abradable seals are one type of seal incorporated into turbines that reduce blade tip clearances.
Abradable seals have been in use in aviation gas turbines since the late 1960's and early 1970's. Abradable seals are a relatively simple means to reduce gas path clearances in both compressors and turbines. The seals offer clearance reductions at relatively low cost.
FIG. 1 shows where abradable materials may be placed. An abradable material 101 is placed on the stationary shroud, ring or casing 103 opposite rotating blade 105 tips 107 to reduce clearances with minimum risk to the turbine components during blade-to-housing rubs. The seals 101 are worn-in by the rotating blade during service with little wear of the blade tips. The seals reduce operating clearances by allowing tighter cold-build clearances without fear of damaging blade tips by tip/shroud closures during turbine transients and at steady state. Additionally, applying an abradable material further reduces effective clearances for often encountered casing out of roundness and rotor lateral movement.
Abradable seal development is materials centered. The material types are typically classified according to their temperature capability. For use temperatures above 760° C. (1,400° F.), porous ceramics are generally used as the abradable material. The most widely used material is a yttria-stabilized zirconia (YSZ). A fugitive polymeric phase is usually added to produce the desired level of porosity. To prevent blade tip wear, a cutting element may be added to the blade tips.
The coating microstructure and porosity are also important for abradability. The more porous the coating is, the higher its abradability but the lower its erosion resistance.
Shown in FIG. 2 is an example of a typical, metallic abradable seal 201. The abradable seal is segmented, made from a number of seal segments 203 where each seal segment 203 is coupled to an inner surface 205 of a turbine blade shroud segment 207. The abradable seal segments 203 are brazed to each shroud segment 207. The seal/shroud segments 209 are coupled together forming a complete shroud 301 having an abradable seal 303 around its inner periphery as shown in FIG. 3. This method of abradable seal construction is compromised in sealing efficiency due to the individual seal segments and difficulty of repair due to brazing.
Cold spray deposition is an emerging technology where solid powder particles at or near room temperature are accelerated to velocities in the range of 500 to 1,500 m/sec in a supersonic gas jet. Upon impact with a work piece surface, these high-velocity cold particles plastically deform and bond with the underlying material by a process similar to explosive welding, but on a much smaller scale. Cold spray deposition can be used to deposit a wide range of metals and other ductile materials at high production rates.
The cold spray process offers many advantages over other metallization processes. Since the powders are not heated to high temperatures, no oxidation, decomposition, or other degradation of the feedstock material occurs. Because the process occurs essentially at room temperature, copper, aluminum, and many other reactive metals can be cold sprayed in an open-air environment with little or no oxidation.
Powder oxidation during deposition is also controlled since the particles are contained within the accelerating gas stream. Other potential advantages include the formation of compressive residual surface stresses and retaining the microstructure of the feedstock. Also, because relatively low temperatures are used, thermal distortion of the substrate will be minimized. Because the feedstock is not melted, cold spray offers the ability to deposit materials that cannot be sprayed conventionally due to the formation of brittle intermetallics or a propensity to crack upon cooling or during subsequent heat treatments.
Abradable material porosity may be controlled by varying the cold spray process parameters. These parameters include composition of the carrier and main gas, spray nozzle travel speed, powder feed rate, main gas temperature and pressure, nozzle spray distance, and powder composition.
It is a challenge to form high performance abradable seals while controlling the porosity of the seal material. It is therefore desirable to have a method that allows for the precise application of abradable seal material to seal backing materials to form air seals that increase efficiency and performance.